Casus belli

Casus belli is a Latin term meaning case of war. In the game, a country with a casus belli (commonly abbreviated to "CB") on another country is deemed to have a valid cause for war. When declaring war, the player is presented with a list of available CBs. They may pick a CB from the list or choose to declare war without a casus belli and accept the penalties of +30 aggressive expansion and −2 stability (halved with full Diplomatic ideas).

Each casus belli has a particular war goal associated with it, generally either controlling a particular province or winning a certain proportion of battles. Meeting the war goal will cause the war score of whichever side has achieved it to gradually rise (up to a max bonus of 25% at a rate of +0.4% per month). The casus belli also determines what peace terms may be demanded at no or reduced Diplomatic Power cost; additionally, some CBs disallow some peace terms entirely. Extracting additional concessions beyond those of the original CB (unjustified demands) will cost diplomatic power to be demanded.

List of casus belli

War goals

Each CB has an associated war goal. This describes what each side has to do to get ticking warscore, i.e. additional warscore over time from having accomplished the goal. Wargoals include:

• Take province / Defend province: Control a specific province. This may be a province chosen on declaring the war, or the capital. Each side gets ticking warscore as long as they control the target. Neither side gets ticking warscore if a third party (e.g. rebels) controls it.
• Show superiority: Have at least 10% warscore from battles. No side gets ticking warscore if neither has said warscore from battles
• Blockade ports: Amount and rate of ticking warscore scales by the proportion of blockading.

Triggered

A triggered casus belli appears when certain actions are performed (like insulting other countries), and can also be given by events and missions. Triggered casus belli only last for a certain amount of time before they expire and are no longer valid. Without Art of War, none of these CBs can be used by an overlord on a subject's behalf.

Casus belli	Wargoal	Duration	Aggressive expansion modifier	Prestige modifier	Warscore cost modifier	Acquisition	Affected peace offers
Restoration of Union	Take capital	240 months	100%	100%	100%	Given to senior personal union partner when a junior partner becomes independent. Cannot be used by the revolution target.	Form personal union
Defected Province	Take target province	12 months	100%	200%	50%	When a province defects due to rebels	Cede province Only on provinces that defected from the aggressor in the past 12 months
Canceled Loan	Superiority	12 months	100%	200%	75%	Given to debtor when creditor defaults on a loan This used to be such an easy way to get a CB that AI always refuses loans since 1.4 (except Indulgent rulers).	Gold Concede defeat Transfer trade power
Discovered Spy	Superiority	12 months	100%	200%	75%	Given by two Japanese events involving ninja	Revoke core Gold Concede defeat
Disloyal Vassal	Take capital	120 months	100%	200%	50%	Given to overlord when a vassal breaks free	Demand vassalization
Imperial Liberation	Take target province	60 months	100%	200%	75%	Given to the Emperor when he demands unlawful territory and the other country refuses	Cede province Only on HRE provinces with the "Unlawful Territory" province modifier Release vassal Release country Return core
Diplomatic Insult	Superiority	12 months	100%	200%	75%	By event and the "Send Insult" diplomatic action	Revoke core Gold Concede defeat Humiliation
Dishonored Call	Superiority	36 months	100%	200%	75%	When an ally refuses a call to arms	Revoke core Gold Concede defeat Humiliation
Subjugation	Take capital	120 months	100%	200%	50%	From vassalization missions	Demand vassalization
Obscure Documents	Take capital	12 months	100%	100%	150%	By Russian "Time of Troubles" disaster event "False Dimitri Overthrown"	Form personal union
Religious Unity	Take capital	24 months	100%	200%	75%	Given to the emperor when an HRE member refuses the emperor's demand for religious conformance	Force change religion
Border Friction	Take target province	12 months	75%	100%	75%	Some events Border friction events generally give claims. One that gives this CB is the "Cossack Aggression" event chain.	Cede province Only on neighbouring non-capital provinces Concede defeat
Trade War	Superiority	24 months	100%	200%	75%	By event Not to be confused with Trade Dispute or Trade Conflict.	Revoke core Gold Concede defeat Transfer trade power Can't: Cede province Return core Revoke core Release vassal Liberate country Force vassalization Force tributary status Transfer vassals Form personal union
Trade Conflict	Blockade ports	24 months	100%	100%	100%	Via the "Justify Trade Conflict" covert action. Not available to Merchant Republics (they get their own version, see below).	Gold Concede defeat Transfer trade power Costs 50% less warscore Can't: Cede province Return core Revoke core Release vassal Liberate country Force vassalization Force tributary status Transfer vassals Form personal union
Trade League Dispute	Blockade ports	24 months	100%	100%	100%	Available to the leader of a trade league against countries they could otherwise justify a trade conflict against.	Gold Concede defeat Transfer trade power Costs 50% less warscore Can't: Cede province Return core Revoke core Release vassal Liberate country Force vassalization Force tributary status Transfer vassals Form personal union
Annexation War	Take capital	24 months	50%	100%	100%	Never This CB is broken and doesn't work properly, nor can it be obtained normally. It could have been a colonial conquest CB.
Government Form War	Take capital	120 months	75%	100%	100%	By event (Milan's neighbours when it becomes an Ambrosian Republic)	Force change government type Cede province Only on core, claimed, or neighbouring provinces
Forced ally to break alliance	Superiority	120 months	100%	200%	75%	Given when a Great Power forces an ally to break their alliance with the country.	Revoke core Gold Concede defeat Humiliation

Automatic

An automatic casus belli is given when certain conditions between you and the defender are met. These last as long as the condition is still satisfied. The "advanced casus belli" are Imperialism and Nationalism and are gained at diplomatic technology 23.

Casus belli	Wargoal	Conditions	Aggressive expansion modifier	Prestige modifier	Warscore cost modifier	Affected peace offers
Humiliate Rival	Take capital	Is the country's rival Attacker not revolution target	100%	125%	100%	Revoke core Gold Annul treaties Transfer trade power Humiliate May Show Strength Costs 100% warscore and awards 30 power projection and 100 of each monarch power. Can't: Cede province Return core Release vassal Release country Force vassalization Transfer vassals Force tributary status Force change religion Concede defeat Revoke electorate Form personal union
Conquest	Take target province	Defender holds province with attacker's claim Attacker not revolution target With Art of War, may be used by an overlord on a subject's behalf.	100%	100%	100%	Cede province Only on claimed provinces (0% cost)
Reconquest	Take target province	Defender holds province with attacker's core Attacker not revolution target With Art of War, may be used by an overlord on a subject's behalf.	25%	100%	75%	Cede province Only on provinces attacker has a core on (0% cost) Return core (0% cost)
Independence	Defend capital	Attacker is a subject nation of defender, other than a colonial nation or tributary Attacker government form is not Daimyo, or defender government form is not Shogunate Attacker is not revolution target	attacker: 100%	100%	75%	Concede defeat Independence
			defender: 50%			Cede province
Colonial Independence	Defend capital	Attacker is a colonial nation of defender Attacker is not revolution target	attacker: 100%	100%	75%	Concede defeat Independence
			defender: 50%			Cede province
Nationalism	Take target province	Attacker is not a subject nation, or is a tributary Attacker has diplomatic technology level 23 Defender holds a province in attacker's culture group Attacker is not revolution target	50%	150%	50%	Cede province Only on provinces in attacker's culture group
Imperialism	Take capital	One of: Attacker has diplomatic technology level 23 Attacker has Invasion modifier (only for Jan Mayen in an unmodded game) Attacker is not a subject nation, or is a tributary Attacker is not revolution target Attacker does not have government reform 'War Against the World Doctrine' enabled	75%	100%	75%	Cede province (0% cost) Return core (50% cost)
War Against the World	Take capital	Attacker has diplomatic technology level 23 Attacker is not a subject nation, or is a tributary Attacker is not revolution target Attacker has government reform 'War Against the World Doctrine' enabled (requires Pirate Republic goverment)	75%	100%	75%	Cede province (0% cost) Return core (50% cost)
War for the Emperor Subject Daimyo attacking the Shogun	Take capital	Attacker is a vassal or junior personal union partner of defender Attacker government form is Daimyo Defender government form is Shogunate Attacker is not revolution target	50%	100%	100%	Cede province Only on claimed provinces
War for the Emperor Independent Daimyo attacking the Shogun	Take capital	Attacker is not a subject country Attacker government form is Daimyo Defender government form is Shogunate Attacker is not revolution target	50%	100%	100%	Cede province Only on claimed provinces
Annex Daimyo	Take capital	Attacker government form is Shogunate Defender government form is Daimyo Defender owns at least 10 provinces, or is independent Attacker is not revolution target	50%	100%	100%	Cede province Only on claimed provinces
Sengoku	Take capital	Attacker and defender are both subject countries Attacker and defender both have government form Daimyo Attacker neighbors defender Attacker is not revolution target	50%	100%	100%	Cede province Only on claimed provinces
Colonialism	Take target colony	Attacker has a colonial claim against defender. This means: Attacker or their subject colonial nation owns land that is overseas for attacker Defender owns land that is overseas for them or they are a colonial subject These respective provinces neighbour each other Attacker is not revolution target	25%	100%	50%	Cede province Only on the provinces that trigger the CB
Liberation	Take capital	Not available, only used in historical wars	100%	300%	75%	Release vassals Release country Return core
Holy War	Superiority	Attacker neighbours defender Attacker and defender belong to different religion groups One of: Attacker has Religious idea "Deus Vult" Attacker is Catholic, and the Curia controller has declared a crusade on defender Attacker is not revolution target	75%	125%	100%	Cede province
Defender of the Faith	Defend country	Attacker neighbours defender Attacker is Defender of the Faith Attacker and defender belong to different religions Attacker has Religious idea "Deus Vult" Attacker is not revolution target One of: Peace of Westphalia has not occurred (Always true if Art of War is not active.) Neither attacker nor defender are members of the Holy Roman Empire	100%	200%	75%	Release vassal Release country Return core Revoke core Gold Concede defeat
Cleansing of Heresy	Superiority	Attacker neighbors defender Attacker and defender belong to the same religion group Attacker and defender have different religion Attacker has Religious idea "Deus Vult" If Art of War is active and attacker and defender are both in the HRE: the Peace of Westphalia has not occurred Attacker is not revolution target	75%	200%	100%	Force change religion Cede province
Excommunication War	Superiority	Attacker is Catholic and not excommunicated Defender is Catholic and is excommunicated Attacker neighbors defender Attacker is not revolution target One of: Peace of Westphalia has not occurred (Always true if Art of War is not active.) Neither attacker nor defender are members of the Holy Roman Empire	50%	125%	100%	Release vassal Release country Return core Cede province Only on core, claimed or neighbouring provinces owned by excommunicated Catholics Revoke core Force vassalization Gold Concede defeat
Trade Dispute	Superiority	One of: Defender embargoes attacker, but attacker does not embargo defender Both countries' government form is Merchant Republic Attacker is not revolution target	100%	200%	75%	Revoke core Gold Concede defeat Transfer trade power Can't: Cede province Return core Revoke core Release vassal Transfer vassals Liberate country Force vassalization Force tributary status Form personal union
Trade League Dispute	Superiority	Attacker is leader of a trade league Attacker is not embargoed by defender A member of attacker's trade league is embargoed by defender Attacker is not revolution target	100%	200%	75%	Revoke core Gold Concede defeat Transfer trade power Can't: Cede province Return core Revoke core Release vassal Liberate country Force vassalization Transfer vassals Force tributary status Form personal union
Imperial Ban	Take target province	Attacker is Holy Roman Emperor The first imperial reform has been enacted Defender is not in HRE, but holds an HRE province Attacker is not revolution target	100%	200%	75%	Release vassal Release country Return core Cede province Only on HRE provinces held by non-HRE country. Cannot demand non-HRE provinces even for normal cost
Coalition	Superiority	In coalition against defender Defender is not already in a coalition war Coalition has at least 3 members Attacker's opinion towards defender is affected by aggressive expansion Attacker is not revolution target	10%	100%	75%	Release vassal Release country Return core Revoke core Gold Concede defeat Cede province Only on core provinces Attackers get +30 war enthusiasm with this CB.
Claim on Throne	Take capital	Attacker has a succession claim on defender Attacker is not revolution target	attacker: 100%	200%	140%	Form personal union
			defender: 50%	100%	75%	Cede province Only on core provinces
Tribal Conquest	Superiority	Attacker neighbors defender Attacker government form is Steppe Nomad Defender government form is not Steppe Nomad Attacker is not revolution target	75%	100%	100%	Cede province Force vassalization Force tributary status Gold
Tribal Feud	Take target province	Both countries' government form is Steppe Nomad Attacker neighbors defender Attacker is not revolution target	75%	100%	100%	Cede province Force vassalization Force tributary status Gold
Revoke Electorate	Take target province	Common Sense is not active Use "Revoke Electorate" diplomatic action instead. Art of War is not active Apparently it breaks the religious league system. Attacker is Holy Roman Emperor Defender is an elector Attacker and defender have different religion Attacker is not revolution target	100%	200%	75%	Revoke electorate
Liberate Elector	Superiority	Attacker is Holy Roman Emperor Defender has an elector subject Attacker is not revolution target	100%	150%	75%	Release vassal Release country Return core
Trade Protection	Blockade ports	Attacker's home trade node, or one where they have a merchant: Attacker has at least 10% share of trade power Defender's privateers have at least 1% share of trade power Attacker is not revolution target	100%	100%	100%	Gold Concede defeat Transfer trade power Costs half amount of warscore
Crush the Revolution	Take capital	Attacker's capital is in Europe, or attacker neighbours defender The revolution target exists Attacker is not revolution target Attacker is not a subject, or is a tributary Attacker's government form is enlightened despotism, despotic monarchy, feudal monarchy, administrative monarchy, constitutional monarchy, or bureaucratic despotism Attacker is not at war with defender Attacker does not have a truce or alliance with defender Defender is revolution target	50%	150%	50%	Force vassalization Cede province Revoke core Release vassal Release country Force change religion Gold Concede defeat Annul treaties (Attacker only) Dismantle revolution Both sides get an additional +50 war enthusiasm when at war with this CB.
Spread the Revolution	Superiority	Attacker is revolution target Attacker is a player, or attacker's capital is in Europe Defender is not attacker's subject Defender is not a revolutionary republic	50%	150%	50%	Force vassalization Cede province Revoke core Release vassal Release country Force change religion Gold Annul treaties (Defender only) Dismantle revolution
Unify China	Superiority	Attacker is not a subject nation, or is a tributary Defender is not a subject of attacker If the Mandate of Heaven expansion is active: Attacker is the emperor of China Otherwise: Both countries have a primary culture in Chinese group If attacker's primary culture is Manchu, attacker is Manchu or Qing Defender owns at least one province in North China, Xinan, or South China region Defender does not have the "Mandate of Heaven" country modifier Attacker neighbors defender	50%	150%	50%	Attacker: Cede province Only on provinces in the China superregion
						Defender: Cede province Only on provinces in the South China, Xinan or South China region or with culture in the same group as defender
Take Mandate of Heaven	Take capital	The Mandate of Heaven expansion is active Attacker is not a subject country Attacker neighbours defender Defender is the Emperor of China Attacker's religion is in pagan or Eastern group	50%	100%	50%	Attacker: Cede province Take Mandate of Heaven
						Defender: Cede province
Humiliate	Take capital	The Conquest of Paradise expansion is active Attacker's government form is Native Council Attacker does not have a truce with defender Both attacker and defender only own one province Attacker neighbors defender Attacker not revolution target	0%	0%	0%	Humiliate
Religious League	Superiority	The Art of War expansion is active Neither country is already in a league war Defender is Holy Roman Emperor Attacker is league leader Attacker is heretic to HRE Attacker is in opposing league Attacker is not revolution target	50%	150%	50%	Force vassalization Cede province Revoke core Release vassal Release country Force change religion Gold Annul treaties Change HRE religion Both sides get an additional +50 war enthusiasm when at war with this CB.
Support Rebels	Take capital	The Art of War expansion is active Rebel faction that attacker is supporting has risen in revolt against defender Attacker is not revolution target	100%	200%	50%	(Attacker only) Enforce rebel demands
Maya Confederation	Take target province	The El Dorado expansion is active Attacker has Mayan religion Attacker is not a subject nation, or is a tributary Attacker has not enacted all religious reforms Defender is not a subject of attacker Defender is in pagan religious group Attacker neighbours defender	75%	100%	75%	Cede province
Flower Wars	Take capital	The El Dorado expansion is active Attacker's religion is Nahuatl Attacker is not a subject nation, or is a tributary Attacker has not reformed the religion Defender is not a subject of attacker Attacker neighbors defender	100%	200%	50%	Force vassalization
Force Migration	Take capital	The Cossacks expansion is active Attacker neighbors defender Attacker is not a subject nation, or is a tributary Defender is not a subject nation, or is a tributary Defender is migratory nation	100%	100%	100%	(Attacker only) Force migration
Force Tributary State	Take capital	Mandate of Heaven expansion is active Attacker is Emperor of China Defender is not a subject nation Attacker neighbors defender	50%	200%	50%	Force tributary status (0% cost)

Conquest details

Having a claim allows the selection of the targeted province as a wargoal. The covert Fabricate Claims diplomatic action provides a claim on a single province (for details of which provinces can have claims fabricated on them, see Espionage § Covert actions). This is an immediate action that costs a certain number of points of spy network strength, starting at 20 and increasing by 5 spy network points per existing claim. In addition, many missions, events and unification national decisions give permanent or temporary claims, sometimes on a large region. There are also some events, such as Border Friction, that allow claims to be gained.

A temporary claim will expire in 25 years after a nation first gets the claim (permanent claims never expire). A claim can't expire during a war with the province's owner; permanent claims never expire, though they can still be given up in a peace deal. Regardless of CB used, claims provide −10% core creation cost, or −25% for a permanent claim.

Choosing a CB

Conquest is ordinarily the easiest CB to get by simply fabricating a claim, but also the least useful. Taking too many provinces can quickly result in a large coalition being formed against the country due to aggressive expansion (AE). It is still desirable to fabricate claims before fighting, in order to reduce coring cost and time, and the diplomatic point cost.

• Holy War and Cleansing of Heresy work on any heathen or heretic, which also removes diplomatic point cost for taking any province, and are both unlocked by the last Religious idea.
• If you have a special CB due to your government form or religion that allows you to take land, you'll usually be better off using it when possible, since it's tailored to your needs. Claims are still useful in these situations, but not strictly necessary. Daimyo, Maya reforming the religion, the Emperor of China, and steppe hordes all want to conquer land with less AE and maybe warscore cost, while Nahuatl trying to reform the religion want cheaper vassalization. The revolution target wants to Spread the Revolution (in fact it can hardly do much more), while their monarchist neighbours want to Crush the Revolution.
  • Deliberately falling to the Revolution disaster is a common world conquest strategy, because the bonuses of being the revolution target and the reduced warscore and diplomatic power cost of the revolutionary CB, in combination with administrative efficiency from high absolutism, make it possible to conquer huge amounts of land very quickly.
• Note that a Casus Belli only reduces the cost of provinces that fulfill its criteria. For instance, Colonialism has 25% Aggressive Expansion and 50% warscore cost, but only for taking provinces that are overseas for both or from overseas colonial nations. Provinces on the same continent as the target's capital or directly connected to the capital will incur full costs (unless they're a colonial nation). When choosing such a CB, it will list the names of all the applying provinces, or show the number of these provinces if there are too many.
• Also note that the reduced warscore cost also affects options that are set on total province cost. So if a country's warscore cost is just over 100, the reduced cost for being wargoal might just make the country annexable/vassalizable.

If the goal of the war is not to seize territory or force-vassalize, then the choice of CB is less important, since extorting money or releasing nations doesn't cause AE. Conquest is fine to use in such a situation. If the country happens to be a rival and releasing vassals or countries is not desired, the special Humiliate Rival CB is ideally suited for this purpose.

Warfare